Psr (paternal sex ratio) is a supernumerary (of B) chromosome in the parasitic wasp. Nasonia vitripennis, which is inherited through sperm but then causes the paternal chromosomes to condense into a chromatin mass prior to the first cleavage division. The paternal chromosomes are eventually lost, but the psr chromosome survives and is transmitted to the next generation. Normally males are derived from unfertilized (haploid) eggs while females are derived from fertilized (diploid) eggs in this wasp. Thus, the affect of psr is to convert diploid female eggs into haploid eggs which develop into psr males. The system is important for two reasons. First, psr is a discrete genetic element that causes paternal chromosome condensation and inactivation during mitosis in the fertilized eggs. It is therefore a tractable system to study genetics of chromosome imprinting and chromosome inactivation. Second, psr is the most extreme example of a "selfish" genetic element so far described. Therefore, genetic studies of psr will increase our understanding of how these elements evolve and function. Research will be undertaken to answer tow questions. (1) What is the genetic structure of the psr chromosome? and (2) How does it cause supercondensation of the paternal chromosomes? These studies are facilitated because (a) we have recombinant clones containing psr specific DNA which can be used as markers to characterize the chromosome by deletion analysis, (b) the gene(s) responsible for supercondensation (spc) occur in a specific region of the psr chromosome (which can be deleted) and (c) genetic experiments indicate that these genes act sometime during spermatogenesis. The following research program will be undertaken. Genetic, cytogenetic and molecular experiments will determine the structure of the psr chromosome and locate the spc gene(s) Other experiments will determine more precisely the timing of psr modification of the paternal chromosomes and search for possible target sites of psr action on the normal chromosomes. A cDNA testes library will be screened to identify psr specific transcripts and to determine if these correspond to the spc gene(s). Besides providing basic insight into mechanisms of chromosomal inactivation and the genetics of "selfish" DNA, the psr chromosome could have significant applications as a biological control agent of insects deleterious to human health and nutrition.